poor mpg

[fastjet]

Well, none of the brakes are binding and the change in mpg has been over the last 6 weeks. Could the change in air temp be responsible?

[Peter.N.]

Yes, but not that amount I wouldn't think. The air temperature does have a significant effect on the fuel consumption, primarily because the air is more dense, thus increasing drag, it probably knocks it down about 10% from very warm to very cold.

Is your water temperature normal? If not a faulty thermostat could result in a cold engine, that will also affect the consumption.

There is this strange anomaly that causes two identical engines to use grossly more or less fuel, I had a 2.2 Hdi C5 mk 1 which was so bad I changed it for a 2.0. which was far better, and yet my son has a 2.2 Hdi 406 Coupe which is completely different. When I drove it home for him from Telford it averaged well over 60 mpg but mine struggled to reach 50 mpg. It doesn't do that consumption when he is driving but its still not bad.

Peter

[moizeau]

Wallop it,

[fastjet]

Update on the mpg for those interested.
I have put a bottle of forte injector cleaner through as suggested. The car definitely drives better for it, smoother, quieter and it is more responsive. So I would say that there was definitely an injector issue. The mpg is now around 35 mpg in town/stop start driving. Mixed would appear to be around 40 mpg and on a quiet run 50 mpg cruising at 50/55 mph. Definitely better than it was but you really do have to be very light on the throttle to get 50 mpg.
Thank you to everyone for their suggestions.

The mpg does seem to be very temperature sensitive, so I was thinking about changing the MAF to see if it helps. A good idea?

[moizeau]

Should be getting better than that, but it's on the up. I'm averaging 43 round lanes and A roads in a 136 auto

[Data]

Well, none of the brakes are binding and the change in mpg has been over the last 6 weeks. Could the change in air temp be responsible?

As said above, changes in temperature will affect how many miles to the gallon you obtain. Your engine takes longer to warm up, the fuel system will therefore have to supply more fuel during warm up for much longer, than for hot running, thus reducing mpg. Your tyre pressures will go down a bit too again affecting mpg if you don't check them regularly.

I note you have used an injector cleaner. I'd use another bottle as well to make sure you remove as much dirt and carbon from the fuel system and injectors as possible. Then reset you mpg indicator. If you are still not achieving good mpg (and I don't think yours is now too bad) then I'd clean the MAF sensor. Use carb cleaner or brake cleaner. It's an easy job and you may find the car becomes much more responsive and much better with mpg's.

Good luck and let us know how you get on.

[Michel]

Is this calculated mpg by manual methods or by using the fuel readout on the dash?

I ask because the computer on all the 4 C5s I've had read around 5mpg lower than reality.

[Peter.N.]

I hope mine does - it reads about 70 mpg on the motorway

Peter

[myglaren]

I hope mine does - it reads about 70 mpg on the motorway

Peter

I get 999.9 mpg - downhill. Otherwise considerably less
I run a spreadsheet and it ties in very closely with the car's estimation. I get just under 40mpg knocking around here.

[fastjet]

The figure is from the onboard computer, I'll do a manual check and report back.
Then I'll also give the maf a clean and will see if that improves things.
Thanks everyone.

[fastjet]

Ok, the fuel consumption up date as promised.
Over the last 500 miles the car has averaged 39 mpg combined. This figure was calculated and matched the cars mpg reading. The lowest value was 33 mpg town stop start in traffic and the highest was 48 mpg dual carriageway at 50 mph. I'm going to give the maf a clean next and see if that makes a difference over the next 500 miles.

[myglaren]

That's about what I get.
The car is usually driven in a series of many short runs with a rare long one - longest was 80-odd miles last weekend.
The MPGometer seems quite accurate but I always feed the data into a spreadsheet.
Filled up today and it came up with an astonishing 1,148 mpg!
Put the wrong odometer reading in though didneye
It was really 36.46